Amplification and over expression of the ErbB family of receptors and their ligands have been reported in various carcinoma cell lines and human malignancies, including breast cancer. Our goal is to determine the effects of neuregulins (NRGs) on the regulation of expression of the epidermal growth factor receptor (EGFR) and its soluble isoform p110 sEGFR. The specific hypothesis is that EGFR and its soluble isoform are regulated by neuregulins in breast cancer cells. We based our hypothesis on the observations that: 1) neuregulins, ligands for the ErbBS and ErbB4 receptors, regulate genes that are implicated in cancer progression;2) there is a positive correlation in the mRNA expression of EGFR and neuregulin in breast cancer cell lines and in breast tumors;3) EGFR and its soluble isoform p110 sEGFR arise from the same gene, therefore they might be regulated by the same factors. To test our hypothesis we will address the following specific aims: 1. To determine if EGFR and its soluble isoform are regulated by neuregulin in breast cancer cell lines. A panel of breast carcinoma cell lines that express different levels of EGFR, ErbB2 and ErbBS will be treated with different neuregulins isoforms and mRNA levels of EGFR and p110 sEGFR will be measured using reverse-transcriptase real-time PCR. The effect of NRGs on EGFR and p110 sEGFR protein levels will be determined by immunoblot analysis. 2. To determine the mechanisms underlying neuregulin-mediated regulation of the expression of EGFR and its soluble isoform. We will analyze if the effects of NRGs are due to EGFR mRNA stability or by increase in transcription of the gene. We will determine the involvement of ErbBS and ErbB2 in the NRGs induced expression of EGFR and p110 sEGFR. ErbB3/ErbB2 downstream signaling pathways, i.e. MARK and PI3K, will be examined for their role in the NRG-mediated expression of EGFR and p110 sEGFR. EGFR has been identified as important therapeutic target due to its implications in the pathogenesis and progression of cancer. Several modulators of EGFR have been identified;however, the effects of neuregulin on EGFR expression have not been studied. A full understanding of the regulators of EGFR expression is needed to design sound therapeutic approaches for patients with EGFR over expressing tumors.